Proteomic Pathway Discovery Reveals That C/EBPalpha Disperses PML Nuclear Bodies To Recruit Its Own Coactivators in a MAPK-Dependent Manner.

The downregulation of the tumor suppressor protein C/EBPalpha is a critical event during cancerogenesis. In contrast restoration of its expression leads to differentiation and ultimately apoptosis. To investigate the mechanisms mediating the antiproliferative and differentiation-inducing activity of C/EBPalpha, we performed a proteome-wide screen for C/EBPalpha target proteins using 2-D-gel electrophoresis and MALDI-TOF masspectrometry. Besides 80 other target proteins we identified the tumor suppressor PML as a protein being regulated by C/EBPalpha. In fluorescence microscopy studies we could show that the induction of C/EBPalpha leads to a dramatic decrease in the number of PML nuclear bodies. We found out that these changes occur due to a reduction in the slower migrating sumoylated isoforms of PML. In reporter assays overexpression of PML was able to increase the trans-activation capacity of C/EBPalpha. CBP was further augmenting this coactivating effect. Cadmium, which is like C/EBPalpha desumoylating the PML protein and dispersing the PML nuclear bodies was having the same activating effect. We could show that p38 was mediating the desumoylation of PML, while the p38 inhibitor SB203580 could partially abrogate the C/EBPalpha-induced effects on PML nuclear bodies. Taken together our investigations could identify a mechanism how C/EBPalpha is targeting PML and CBP to sites of active transcription.